1. Field of the Invention
This invention relates to protection of a solid state power amplifier (SSPA) from large peak, short duration noise signals. More specifically, the SSPA uses a limiter which limits the noise signals and controls drive level related phase shift.
2. Description of the Related Art
An SSPA is used for communication applications in frequencies ranging from a few dozen Mhz to 10 Ghz. The SSPA output section's power transistors are subject to damage or degradation if exposed to large peak value, short duration input noise mixed with the communication signals.
One protection technique senses the SSPA output power level and feeds the signal back through an integrating comparator to drive an attenuator in the signal path prior to the output power transistors. This closed loop negative feedback protection circuit, commonly called automatic level control (ALC), maintains the average amplitude of the signals applied to the output power transistor to a safe level. The time constant of the ALC loop is selected to be very large compared to the signal bandwidth to stabilize the feedback protection system and prevent degradation of the SSPA linearity performance. This averages the response of the protection characteristics, rather than responding to shorter duration noise signals (short compared to the loop time constant). The problem with the ALC technique is that input signals containing large peak to rms content such as noise are not controlled and can damage the output power transistors, since the protection circuit is responding to an average rather than to peak noise conditions.
Another protection technique uses a limiter in the SSPA input stage to limit the maximum value of large peak value, short duration noise. However, these input limiting stages have non-ideal characteristics which cause the limiter either to degrade the amplifier linearity performance or reduce the amount of protection provided.
What is needed is a solid state power amplifier in which the output power transistors are protected from transient overdrive conditions whole simultaneously preserving the SSPA linearity.